The number of man hours devoted to assembly seldom occupies less than 10% of the labor force in any industry and may require more than one third of the total work force in some instances. In an effort to improve manufacturing productivity, increasingly concentrated efforts have been undertaken to design automated devices capable of assembling mating parts or components in applications such as inserting a pin or bolt into an orifice, shafts into bearings, bearings into bores and similar operations. Advanced automated assembly devices or robots may be programmable, and have been utilized to assemble more complicated items such as alternators for vehicle engines and similar "stack" products where all component parts can be added from a single direction. Many of the devices developed to date share the advantages of requiring no separate energy sources, no people, and no sensors or servos for operation.
Regardless of the sophistication of assembly machinery, difficulties have been encountered in accommodating misalignment between mating parts particularly for small clearance fits of low aspect ratio using machine or tooled assembly methods. In any system which does not use manual tactile feed-back (feel) and manual manipulation, such insertion is a problem. Whether resulting from the design of the mating parts, through normal process variations or by unavoidable error, misalignment between mating parts can create wedging, jamming and damage to the parts during the assembly operation. In addition, the mechanisms which deliver either one or both of the mating parts to a position for assembly may not provide precise registration of such parts relative to each other which could create further misalignment. Thus, robots, manipulators, and other automated assembly devices which do not depend on skillful manual manipulation techniques, must include means to accommodate misalignment between mating parts to avoid jamming or wedging and reduce excess insertion loads.
U.S. Pat. Nos. 4,098,001 to Watson and 4,155,169 to Drake et al are examples of prior art insertion compliance systems designed to accommodate misalignment between mating parts during assembly without manual manipulation and without excess insertion loading. The devices described in both of these patents are directed to the problem of inserting a pin into a bore. Translational flexible elements and rotational flexible elements having specific compliances are positioned in a particular geometrical relationship to project a center of compliance or elastic center at or near the end of the pin to be inserted within the bore. To achieve optimal performance, the systems of Watson and Drake et al require relatively precise positioning of the flexible elements and control of the magnitude of their compliance characteristics, particularly the rotational flexible elements, to accurately project the center of compliance to the desired location along the pin. Such design criteria increases the expense of the devices both in terms of material and fabrication costs. In addition, since the position of the flexible elements and their compliance characteristics are fixed, the location of the center of compliance is also fixed. This means that in order to obtain an optimal level of effectiveness in an assembly operation, the position of the insertion end of each part to be assembled must be the same. Therefore, the utility of such devices is significantly limited in assembly operations where the automated equipment is required to assemble a variety of parts having different length dimensions.